1. Field of the Invention
The present invention relates to an image capture device for use with a scanner, a video camera, a digital still camera, etc.
2. Description of the Related Art
Image capture devices called “CMOS sensors” utilizing CMOS (complementary metal-oxide semiconductor) process have received attention in recent years. The CMOS sensors are expected to be especially useful in the field of portable information apparatuses because they can be easily loaded with peripheral circuits and they can be driven at relatively low voltages.
Japanese Laid-Open Patent Application No. 2-296470 discloses a photoelectric converter that has an amplification stage between pixels and storage capacitors for the purpose of attaining an increased S/N (signal-to-noise) ratio of the CMOS sensor. FIG. 7 is a diagram showing an equivalent circuit of the photoelectric converter disclosed in Japanese Laid-Open Patent Application No. 2-296470. In FIG. 7, MOS transistors M1 and M2 constitute a grounded-source amplifier. The grounded-source amplifier amplifies pixel signals input to the gate of the MOS transistor M1 and supplies the amplified pixel signals to storage capacitors CT1 and CT2. The storage capacitors CT1 and CT2 temporarily store the amplified pixel signals. Thus, pixel signals are amplified before being stored in the storage capacitors CT1 and CT2. Accordingly, the photoelectric converter can be prevented from being affected by circuit noise occurring at output lines S1 and S2 and subsequent stages. Therefore, the S/N ratio of the CMOS sensor can be improved.
For example, in a typical image capture device, an output amplifier is provided next to the output lines S1 and S2 so as to drive an external load. In this case, the output amplifier is required to exhibit wide bandwidth capability because it needs to follow high readout frequencies. Therefore, the output amplifier is a potential source of large noise. However, providing the grounded-source amplifier as described above can dramatically reduce the influence of noise generated at the output amplifier.
In Japanese Laid-Open Patent Application No. 2-296470, the photoelectric converter has two storage capacitors CT1 and CT2. Although not specifically described in Japanese Laid-Open Patent Application No. 2-296470, the two storage capacitors may be configured to store different outputs, as disclosed, for example, in Japanese Patent Publication No. 8-4127. More specifically, one storage capacitor stores an output corresponding to a reset state of every pixel, and the other storage capacitor stores an output corresponding to a read-out photoelectric signal. Then, the two outputs are subjected to difference processing at the subsequent stage, so that noise caused by a dark signal error of every pixel can be removed to some extent.
In the above-described photoelectric converter disclosed in Japanese Laid-Open Patent Application No. 2-296470, the stage subsequent to horizontal output lines obtains a difference between an output corresponding to a reset state of every pixel and an output corresponding to an image signal. Thus, a single final-stage circuit performs difference processing on signals from all pixels. This may decrease processing speed in some cases. In addition, a problem arises in that it is impossible to employ a difference circuit requiring long processing time.